The present invention relates to flowmeters, such as vortex shedding meters or swirlmeters which are responsive to a fluid flow. In particular, it relates to electronics for such meters which reduce noise in a flow responsive signal.
Flowmeters sense the flow of liquid and gases in conduits and produce a noise contaminated flow responsive signal. Under certain circumstances, the presence of an obstacle or shedder in a flow conduit causes periodic vortices. A vortex flowmeter produces shedding vortices from a bluff body. The frequency of these vortices is directly proportional to the flow velocity in the meter. The shedding vortices produce an alternating differential pressure across the bluff body at the shedding frequency. This differential pressure is converted to an electrical signal by piezoelectric crystals or other differential pressure devices. The magnitude of the differential pressure or electric signal is proportional to .rho.V.sup.2, where .rho. is the fluid density and V is the fluid velocity. When the ratio of pipe diameter to the size of the shedding bar is held constant, the signal is proportional to .rho.D.sup.2 F.sup.2, where D is the inside diameter of the meter and F is the shedding frequency. The flowmeter produces pulses having a frequency proportional to the flow rate. The swirlmeter produces a similar flow responsive signal by measuring the vortex precession frequency produced by swirling the flow, then passing the flow through a downstream contraction and expansion.
The vortex flowmeter signal comprises a fundamental signal which has a fundamental frequency representative of the flow and an associated noise signal at various frequencies caused by fluid turbulence and other unrepeatable factors such as pipe vibrations, common mode pressure variation and noise from acoustic sources. Pipe vibrations caused by pumps, motors and unsupported sections of pipe are usually in the 0 to 100 Hz range and common mode pressure noise in the 10 to 1000 Hz range, while acoustic noise is generally above 100 Hz. Fluid turbulence results in noise on both sides of the fundamental frequency. Because fluid turbulence noise generally increases in amplitude as the flow velocity increases, it is particularly troublesome when low frequency turbulent noise below the fundamental frequency is disproportionally amplified by signal processing electronics.
Flowmeters, like vortex shedding meters and swirlmeters, are designed for a variety of applications encompassing wide ranges of flow rates, pipe diameters and fluid densities. Consequently, such meters operate over a relatively large dynamic range. When fluid density is constant, the flow velocity range is typically 25 to 1. Even with the flow range at 25 to 1, signal amplitude will change by a ratio of 625 to 1, because the signal is proportional to the square of the velocity. When variable fluid densities are taken into account, which range between 1 and 800 in fluids such as atmospheric air to liquids, a frequency change of 100 to 1 is possible for a specific meter size and will result in a maximum signal amplitude range from 10,000 to 1. Unfortunately, the signal-to-noise ratio changes markedly over the ranges. Furthermore amplitude and frequency modulation of the flow signal introduces low and high frequency noise that causes problems for some signal conditioning systems. Consequently, a single filtering system is needed which improves variations in signal to noise ratio over a large dynamic range encompassing wide ranges of flow, density and diameter yet differentiates flow signal from unwanted noise.
In order to provide electronics to improve the signal to noise ratio over a large dynamic range, some electronics control the amplitude of the vortex signal through a feedback method. Other systems utilize a phase lock loop that averages phase errors. Both amplitude control and phase lock loop systems have a response time set to the lowest frequency or data rate. As a result, high frequency response suffers.
A method of using signal amplitude of the vortex shedding differential pressure signal to determine mass flow without a separate density measuring method is known. This method has several desirable characteristics, such as allowing calculation of mass flow without additional measuring instruments. Further, the method is independent of the fluid composition. A method for demodulating the AC mass flow signal into a DC signal is known. The method suffers from slow response time and problems with the inherent amplitude and frequency jitter of the vortex signal. It also requires an amplifier with a low pass characteristic. This amplifier is subject to significant temperature errors, increasing the low frequency noise due to its derivative function, as well as the ranging problems discussed above.